In a farm tractor it is desirable to provide the operator not only with steering capacility under all conditions, even when the engine is dead, but also with responsive steering at low engine speeds. In order to be responsive, i.e. to respond quickly to manual rotation of the steering wheel, the hydraulic system must be capable of supplying a predetermined minimum flow of hydraulic fluid under pressure to the steering system. Since the output of the pump in the hydraulic system is directly related to the speed of the engine driving the pump, it is obvious that the pump flow at the lowest engine speeds normally encountered during operation of the tractor will determine the poorest response in the steering system.
When a fixed displacement pump is used it should be sized so that its output at the aforementioned lowest speed will meet the predetermined minimum flow. Such a pump will be oversized for normal operation and at all but the lowest engine speeds will pump an excessive volume of fluid, which heats the hydraulic fluid necessitating larger oil coolers to maintain proper operating fluid temperatures, will generate higher noise levels, will require larger conduits connecting the steering system with the pump and reservoir, and will require more input horsepower resulting in higher fuel costs.
Use of a variable displacement pump would overcome many of the aformentioned problems regarding the use of a fixed displacement pump. However, variable displacement pumps are seldom used in open-center hydraulic systems, because the compensator would tend to urge the pump toward maximum when, in reality, there was no need for additional pump output. The open-center type hydrostatic steering system is desirable because it is available commercially and because it more readily permits steering capability with the engine dead, and is easily adapted to utilize pump output for actuation of other hydraulic apparatus and for needed lubrication of critical tractor components.
It is also of significance that the recent trend is to tractors that steer by articulation of the frame. Such steering arrangements usually require a pair of steering cylinders having relatively large displacement volumes. The desired responsive steering in such tractors necessitate even higher minimum flows than is the case with comparably-sized tractors utilizing the more conventional two-wheel, Ackerman type steering.
It is, therefore, an object of this invention to provide a hydraulic system which overcomes most of the disadvantages normally associated with a fixed displacement pump, while permitting the use of a conventional open-center type hydrostatic steering system.
Further objects of this invention are to provide a hydraulic system which is relatively quiet in operation, which minimizes the size of conduits utilized therein, which is economical to construct and to operate, which minimizes heating of the hydrauic fluid during operation, and which permits use of common parts from open-center type hydraulic systems.